1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device that can electrically rewrite data and a method of manufacturing the same.
2. Description of the Related Art
Resistor elements have been widely used in analog electric circuits. Examples of the analog circuit using a resistor element may include an RC delay circuit, a reference potential generation circuit, a regulator of voltage booster, and a regulator in a voltage down-converter. In particular, in a nonvolatile semiconductor memory device such as a NAND-type flash memory, various internal voltages are generated. Accordingly, the nonvolatile semiconductor memory device has a plurality of analog circuits mounted thereon and uses a large number of resistor elements.
As the resistor elements used in the nonvolatile semiconductor memory device, three types of resistor elements, i.e. (A) a resistor element using a diffusion layer, (B) a resistor element using polysilicon (for example, refer to Japanese Patent Application Laid-Open No. 2007-266499), and (C) a resistor element using a metal wiring line (for example, tungsten) having a relatively high resistance value are mainly used. A sheet resistance value of the resistor element (A) is the highest, that of the resistor elements (B) is lower than that of the resistor element (A), and that of the resistor element (C) is lowest. The diffusion layer of (A) can provide a resistor element that has a high resistance value and a small area. However, since a width of a depletion layer that is generated in an end of the diffusion layer varies depending on an applied voltage, the area of the diffusion layer related to electric conduction varies according to the variation in the width of the depletion layer. That is, the resistance value depends on the applied voltage. This characteristic is not preferable as a characteristic of a resistor element. That is why the resistor element (A) is rarely used in an analog circuit that needs a resistance value with high precision. The resistance value of the resistor element (C) does not depend on the voltage. However, since the resistor element (C) is made of metal and has a low resistance value, an area to achieve a large resistance value is large. Under these circumstances, though a resistor element (B) using the polysilicon has an area relatively wider than that of the resistor element (A), the resistor element (B) using the polysilicon is frequently used in an analog circuit.
In manufacturing processes of the nonvolatile semiconductor memory device, a change in a process such as a thermal process is sometimes conducted for, improvement of a yield, reliability of a device, and a characteristic of a device. For this reason, the resistance value of the resistor element formed of polysilicon may be greatly changed. That is, an operation of a circuit using the resistor element cannot be ensured. In order to solve the above-described problems, a mask used at the time of etching the polysilicon needs to be made again. Accordingly, in the resistor element used in the nonvolatile semiconductor memory device, a manufacturing process that can easily change a resistance value is required.